1. Field of the Invention
This invention relates to a method for storing a submarine optical cable in a cable tank situated on land or on board a ship.
A submarine communication system using coaxial cables has been widely adopted as a means of international or domestic communication, since this system is superior to a wireless communication system using, for example, communication satellites, or a short wave communication system, because noise or delay time is reduced and a more reliably confidential communication is ensured. Recently, there have been remarkable developments in submarine optical communication systems using an optical cable, because the transmission capacity thereof is larger than that of coaxial cables.
In a submarine optical communication system, to prevent a deterioration of transmission characteristics, and to amplify or reproduce the optical signals transmitted therethrough, submarine optical repeaters are provided at predetermined intervals, for example, 50 km. Each optical repeater is driven by a constant electric current supplied from a power source incorporated in the optical cable itself.
Usually, a submarine optical cable is very long, for example, 1000 km or more, and therefore, a compact means of storing the submarine optical cable on land or on board a ship is desired.
2. Description of the Related Art
In order to store a long submarine optical cable having a length of, for example, 1000 km or more, on land or on board a ship before it is laid in the sea, the cable must be stored in a compact manner, i.e., in as small a space as possible.
Conventionally, a cable tank 16 is situated on land or on board a ship, as shown, for example, in FIG. 10, and a submarine optical cable 10 is accommodated in the cable tank 16 in such a manner that it is spirally wound as a coil to form a plurality, for example, several tens or hundreds, of plane coil layers. On land, the cable tank 16 is constructed under the ground surface, for example, of reinforced concrete, and has a cylindrical coil receiving space defined by a central vertical column 18, a cylindrical peripheral side wall 20, and a horizontal bottom wall 22. In the prior art, the cable 10 is spirally wound in this cable tank 16 in the same direction throughout the entire length thereof, and thus stored in a compact manner.
The submarine optical cable 10 comprises, as shown in FIG. 11, an optical fiber unit 22 including a central core 24 made of copper, a plurality of optical fiber cables 26 spacedly arranged around the central core 24, a silicone rubber portion 28 filled and solidified around the central core 24 and fiber cables 26, inner and outer aluminium layers 50 and 52, which constitute an electrical power supply system, and high-tension resistant cables 54 disposed therebetween.
In the conventional cable storing method, since a submarine optical cable 10 having a length of several hundred kilometers or about 1000 km is spirally wound in the same direction, throughout the entire length thereof, in the cable tank 16 situated on land or on board a ship, the cable 10 constitutes a coil having a self-inductance of several thousands Henry (H) or ten thousand H or more.
Sometimes it becomes necessary to examine the transmission characteristics of the optical cable 10 while it is stored as a coil in the cable tank 16. In that case, an electrical power must be supplied to the optical repeater 12 from the power source 14 through the aluminium layer 50 and 52 of the optical cable 10. Such an examination of the transmission characteristics is carried out while the optical cable 10 is stored in the cable tank 16 situated on land or on board a ship, or when the optical cable 10 is unwound from the tank 16 to be laid in the sea.
Therefore, if a sudden power peak appears when electrical power (constant voltage of a few Amperes) is supplied to the optical cable 10, an extreme change of current may occur and, therefore, an induced high voltage will be generated due to the self-inductance of the coiled submarine optical cable 10. The optical repeater 12 or other such elements connected to the submarine optical cable 10 may be affected and sometimes damaged by this extreme high voltage. Such problems occur not only when a sudden power peak appears at a start of a supply of electrical power, but also when, for example, the cable 10 is disconnected for some reason during the supply of electrical power. In the latter case, a more violent change of current may occur, and thus a higher voltage may be generated due to the self-inductance of the coiled submarine optical cable 10. Thus, there is a possibility that the optical repeaters 12 or the like connected to the cable 10 will be badly damaged.